The Effects of Various Factors on Plant Yield
Yield of commercially valuable species in the natural environment is sometimes suboptimal since plants often grow under unfavorable conditions. These conditions may include an inappropriate temperature range, or a limited supply of soil nutrients, light, or water availability. More specifically, various factors that may affect yield, crop quality, appearance, or overall plant health include the following.
Nutrient Limitation and Carbon/Nitrogen Balance (C/N) Sensing
Nitrogen (N) and phosphorus (P) are critical limiting nutrients for plants. Phosphorus is second only to nitrogen in its importance as a macronutrient for plant growth and to its impact on crop yield.
Nitrogen and carbon metabolism are tightly linked in almost every biochemical pathway in the plant. Carbon metabolites regulate genes involved in N acquisition and metabolism, and are known to affect germination and the expression of photosynthetic genes (Coruzzi et al., 2001) and hence growth. Gene regulation by C/N (carbon-nitrogen balance) status has been demonstrated for a number of N-metabolic genes (Stitt, 1999; Coruzzi et al., 2001). A plant with altered carbon/nitrogen balance (C/N) sensing may exhibit improved germination and/or growth under nitrogen-limiting conditions.
Hyperosmotic Stresses, and Cold, and Heat
In water-limited environments, crop yield is a function of water use, water use efficiency (WUE; defined as aerial biomass yield/water use) and the harvest index [HI; the ratio of yield biomass (which in the case of a grain-crop means grain yield) to the total cumulative biomass at harvest]. WUE is a complex trait that involves water and CO2 uptake, transport and exchange at the leaf surface (transpiration). Improved WUE has been proposed as a criterion for yield improvement under water limiting conditions and drought. Water deficit can also have adverse effects in the form of increased susceptibility to disease and pests, reduced plant growth and reproductive failure. Genes that improve WUE and tolerance to water deficit thus promote plant growth, fertility, and disease resistance.
Yield may also be limited by a plant's intrinsic growth rate. A faster growth rate at the seedling stage could allow a crop to become established faster. This would minimize exposure to stress conditions at early stages of growth when the plants are most sensitive. Additionally, it could allow a crop to grow faster than competing weed species. Accelerating plant growth overall would also improve yield per acre or reduce time to harvest. For example, this would be particularly desirable in forestry: an important aim in tree-breeding programs around the world is to produce plants with increased growth rates and stem volumes, and shorter rotation times.
Perennial Plants and Annual Crops
Perennial plants such as long-lived trees have a life style considerably different from annual plants such as Arabidopsis in that perennial plants such as trees have an indeterminate growth pattern, whereas plants like Arabidopsis eventually stop growth after the plant flowers and sets seed. The final size of an Arabidopsis plant is in many ways dependent on the developmental program from germination to flowering and seed set. Therefore, any change in the timing of these events can drastically change the size of the plant.
Perennial plants also may cycle between periods of active growth and dormancy. During active growth leaves perform photosynthesis to capture energy which then used to drive various cellular processes. The fixed carbon which converted to sucrose is transferred to storage tissues where it is stored during the dormant state. As growth reinitiates after release from dormancy, the fixed carbon is translocated to actively growing tissues. Similarly for nitrogen, amino acids are translocated also to storage tissues and stored as storage proteins during dormancy, and broken down as growth starts. Thus the life cycle of long lived trees differs significantly from annual crops. Due to these differences between annual crops and perennial plants such as trees, determinants of yield and the ability to measure them are likely to considerably different. For example for annual crops, seed size/yield has been proposed to be a measure of plant size and productivity, but this is unlikely to be the case since perennial plants such as trees take several years to flower and thus seed yield, if at all, is only an indicator of growth conditions that prevail during the year the plant flowered. Actually, in many instances a model system such as Populus tremulaxtremuloides is much better for reliably confirming genes that can be used for increasing biomass production. Also the important biomass of trees is usually the wood, this being a tissue not present in many of the commonly used plants model systems such as Arabidopsis. Thus, poplar, which has a small, fully sequenced genome and is phylogenetically related to Arabidopsis, provides an excellent model for studying traits that are unique in woody perennials, giving unique insights into useful trait genes for biomass production and wood quality. A plant's traits, including its biochemical, developmental, or phenotypic characteristics that enhance yield or tolerance to various abiotic stresses, may be controlled through a number of cellular processes. One important way to manipulate that control is through transcription factors—proteins that influence the expression of a particular gene or sets of genes. Transformed and transgenic plants that comprise cells having altered levels of at least one selected transcription factor, for example, possess advantageous or desirable traits. Strategies for manipulating traits by altering a plant cell's transcription factor content can therefore result in plants and crops with commercially valuable properties.
We have thus identified important polynucleotide and polypeptide sequences for producing commercially valuable plants as well as the methods for making them and using them. Other aspects and embodiments of the instant claims are described below and can be derived from the teachings of this disclosure as a whole.